1. Field of Invention
This invention relates to a granular media filtration system, and in particular, to one having an improved method and apparatus for backwashing.
2. Brief Statement of the Prior Art
Granular media filtration systems are used in a large number of applications for the filtration of suspended impurities from water such as waste, industrial water, sewage plant affluent, etc. The filtration systems customarily include a containment vessel in which reposes a bed of granular filtration media, usually granules of anthracite or silica. A liquid containing suspended solids is introduced into the bed, usually in a downflow configuration and the filtered liquid is removed from the opposite side of the bed. When the filtration media becomes clogged with separated solids, the solid bed is regenerated by flushing with water and/or air, usually with a backflow treatment, wherein water and air are introduced beneath the granular media bed and flow upwardly therethrough. The introduction of air and water causes agitation of the granular medium particles releasing trapped previously separated solids. Solids are removed by keeping them in suspension from liquid and air agitation and subsequent removal by dilution into the backwash water. Flotation of the solids can also be another removal mechanism.
The separation is carried out in some installations with a number of trough separators which are placed across the vessel near its upper extremity. The liquid enters the trough and a quiescent zone is maintained within the trough by placing an air deflection baffle beneath an open throat in the bottom of the trough. A collection channel is placed within the separator trough and the liquid within the trough which is clarified of the granular media by its settling in the quiescent zone of the collection channel spills over the upper edges of the trough, carrying with it the low density and fine, separated solids which are then withdrawn from the vessel.
Some difficulties are experienced with the aforedescribed apparatus. There are two major difficulties. The first is that the separated granular media particles which settle in the separator trough tend to accumulate on the upper surface of the air deflection baffle, clogging the separator trough which fills with granular media particles until the separation system is no longer effective. The other major difficulty is the complete exclusion of air from the separator trough. Even small amounts of air will transport significant amounts of media from the filter into the backwash channel.
Accordingly, there exists a need for an effective separation system which does not become clogged with settled granular media particles so that the system can be operated during backflushing over sufficient lengths of time to regenerate the granular media bed for reuse in the filtration system.